Chain-link Multi-level Voltage source converter systems comprise a number of converter cell modules connected in series. Each converter cell module forms one “link” (or cell) of a single phase voltage source converter (VSC) and is switched on/off a number of times during a fundamental frequency cycle of an ac system. The chain-link converter may comprise a number of phases, each phase comprising a chain of such converter cell modules connected in series to each other.
Briefly, the converter cell typically comprises semiconductor assemblies of turn-off type, e.g. insulated gate bipolar transistors (IGBTs). As an example a converter cell module comprises four IGBTs. A free-wheeling diode, also denoted anti-parallel diode, is connected in parallel with each IGBT and conducts in the opposite direction of the IGBT. The IGBTs are connected in an H-bridge arrangement with a DC link capacitor bank.
The number of converter cell modules connected in series for each phase is proportional to the AC voltage rating of the ac system. The number of converter cell modules can therefore be large for high voltage applications having no mains transformers, which entails high costs. Further, the current demands on the converter cell modules may also be low in high voltage systems, resulting in the use of overrated semiconductor components, which again entails high costs.
On the other hand, if the application at hand requires higher currents than available in semiconductor ratings, converter cells or semiconductor switches have to be parallel connected, thus yet again resulting in high costs.
In order to provide flexibility in these types of applications, the use of transformers is a solution. However, the cost of a mains transformer is high and a mains transformer requires a large footprint, not available for all types of applications.
It is necessary that the series connected cells have a feature that allows the cell to be bypassed in case of a failure in the cell in order to enable continuous operation of the converter system without tripping if for example a semiconductor device fails. That is, if the semiconductor device, e.g. the IGBT, is damaged the cell cannot be allowed to go into an open circuit, as the series-connection of converter cell modules then would form an open circuit. There are semiconductor devices available that go into short circuit when damaged, e.g. press-pack IGBTs, which if broken become a short-circuit, a behaviour that is known as short-circuit failure mode (SCFM). However, the cost of such semiconductor devices is very high, even as large as twice the cost of standard industrial type of devices.
The use of standard industrial type of devices requires the use of additional components such as very fast bypass switches that bypass a faulty cell or device. This, yet again, entails high costs.
U.S. Pat. No. 6,236,580 discloses a modular multilevel adjustable supply with series connected active inputs.
The publication “Series 300TM UPS Three Phase 10 kVA to 125 kVA; 60 Hz—Installation, Operation & Maintenance Manual” by Lieber Power Protection discloses a computer protection system comprising a back-to-back AC converter which can be bypassed by a switch for maintenance purposes.
“A voltage sag supporter utilizing a PMW-switched autotransformer”, Power Electronics Specialists Conference, 2004, Pesc 04.2004 IEEE 35th annual, Aachen, Germany 20-25 Jun. 2004, Piscataway, JN, USA, IEEE, US vol. 6, 20 Jun. 2004, pp 4244-4250, by Dong-Myung Lee et al. discloses a distribution-level voltage control scheme that can compensate voltage sag and swell conditions in three-phase power systems. Voltage sag support is based on a Pulse Width Modulated (PWM) autotransformer.